Very little connectivity with iPhone 3GS

Hi, I bought a german Iphone 3 GS from a friend who had used it in a contract with SIM-Lock but that contract expired, she had it unlocked and then sold it to me. The phone accepted my SIM-Card and would let me set up but then it had no connectivity in my room, where my previous phone had FULL reception. I thought the phone might not accept my provider or the antenna was broken or something but I took the thing for a walk and somewhere along the way it must have connected because I got a text message from my girlfriend. However, when I checked it, it had already lost connection again.

Turning Wi-Fi, 3G and/or Bluetooth off didn't change a thing.

The phone was and is fully updated to iOS5 and the new baseband, I didn't try any phishy unlockings or jailbreaks and I had my iTunes reset it. My Genius Bar appointment is not for another week and I figured I could ask around whether anybody has had those problems yet.

Thanks for the help! Simon